The present invention relates to a sheet inspection apparatus for a sheet-fed offset printing press, which detects a misfed sheet in a defective printed state after printing of sheets.
Generally, in a sheet-fed offset printing press of this type, to inspect the printed state of a sheet upon completion of all printing processes, the inspection process is provided after the operation of the impression cylinder of the final printing unit. In this case, when the printed state is to be inspected on the impression cylinder of the final printing unit, the trailing edge of the sheet which has passed between the impression cylinder and a blanket cylinder is pulled to the blanket cylinder side due to the viscosity of the ink on the blanket cylinder, and the sheet largely swings, resulting in so-called instability of sheets. For this reason, determination of properly conveyed sheets and misfed sheets cannot always be exactly performed.
Therefore, as disclosed in Japanese Patent Laid-Open No. 63-64748, a frame is added in a delivery unit continuing to the printing unit. A support plate functioning as a printing paper stabilization means for straining a sheet conveyed by an ejector chain is arranged in this frame in correspondence with the inspection unit such that the sheet is inspected in the sheet conveying unit of the delivery unit.
However, since the inspection unit is set between the impression cylinder of the final printing unit and the delivery unit, an inspection unit adjustment operation or maintenance operation for cleaning paper dust adhering to the detection unit of the inspection unit or anti-setoff powder can hardly be performed, and the maintenance operation takes a long time. Additionally, the sheet conveying unit of the delivery unit has a dryer unit for drying a printed sheet. When the inspection unit is set to be close to this dryer unit, an electronic circuit including electronic components constituting the inspection unit is adversely affected by heat generated by the dryer unit. As a result, the inspection unit erroneously operates to impede to properly inspect the printed state.